<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
       
    </style>
</head>
<body>
    <input type="button" value="选择" id="btn">
    <br>
    <select id="food">
        <option>烤肉</option>
        <option>拉面</option>
        <option>麻辣烫</option>
        <option>小龙虾</option>
        <option>火锅</option>
        <option>外卖</option>
    </select>
    
    <script>
        var btn = document.getElementById("btn");
        var food = document.getElementById("food");
        var opts = food.getElementsByTagName("option");
        btn.onclick = function (){
                var n = Math.floor(Math.random( )*opts.length);
                opts[n].selected = true;
            }
        
    </script>   
</body>
</html>